Older women have as good a chance of having a baby under IVF with one embryo as with two, according to research which may eventually help to eradicate the risk of multiple births.

The study from Finland also found that patients aged between 36 and 39 are just as likely to have a baby from a single embryo as are younger women. The crucial factor was the quality of the embryo. The findings, published in Human Reproduction, were yesterday hailed by fertility experts keen to reduce the chances of multiple births - a risk that occurs in nearly a quarter (23.7%) of all IVF pregnancies.

Only one in 10 women in Britain has single embryo transfers at present, with most opting to have two embryos in the belief this will maximise their chances of carrying a baby. The majority of those over 40 opt for three embryos.

The fact that three-quarters of couples have to pay for their treatment, and that each cycle can cost up to £6,000, also means the vast majority are reluctant to only have one embryo transferred.

Multiple births remain the biggest risk in fertility treatment, causing major problems - such as three to five times the risk of premature birth or cerebral palsy for the babies - as well as an increased burden on the NHS. Figures from Sheffield University suggest the cost for triplets born by IVF was £32,350 a pregnancy, 10 times the amount for a single baby.

Single embryo transfer is used by 60% of women under 36 in Finland, in all women under 35 in Belgium, and in Sweden in all but the most exceptional circumstances. But patient groups and experts stressed this would only happen if Britain offered to fund more treatment.

The researchers analysed 1,224 cycles of treatment using fresh embryos and 828 using frozen embryos in women aged 36 to 39. They compared results for cycles that used just one top-quality embryo with those using a poor embryo and those using two embryos. And they compared the findings with those for women aged under 36 in previous research.

The scientists, led by Hannu Martikainen, from the University of Oulu, found that a third of women (33%) in the older age group became pregnant after one embryo was transferred, compared with 29.9% who had two implanted. The success rate was similar to that of younger women - 31% to 35% - while the live birth rate was also similar at 26% compared to 27% to 30% in the younger group.

They also analysed the likelihood of getting pregnant with more than one cycle - the cumulative pregnancy rate - and found over half (54%) of women were successful with the single embryo technique compared to 35% with two embryos. Dr Martikainen said: "This suggests that embryo quality is the most important parameter. It's quality, not the quantity."

Peter Braude, chair of the scientific advisory committee at the Royal College of Obstetricians and Gynaecologists, stressed that unless the NHS funded two cycles per couple, and included funding for freezing embryos, couples would be reluctant to have just one embryo implanted. The cost of this was far less than the cost of care in neonatal units for premature babies or long-term care for those with cerebral palsy.

Allan Pacey, senior lecturer at Sheffield University and honorary secretary of the British Fertility Society, said: "The success of single embryo transfer in countries such as Finland has only been possible because IVF is so well funded by the state."

Clare Brown, chief executive of Infertility Network UK, said: "You can understand the patients wanting to make the most of the one cycle they can afford, or that they can obtain on the NHS."

FAQ IVF Treatment

How does IVF work? Eggs recovered from the woman after stimulation by drugs are fertilised with sperm in a dish. The resulting embryo or embryos are placed in the woman's womb two or three days later. Older women tend to produce fewer eggs, and so there are fewer embryos from which to choose.

How does an embryologist choose a top-quality embryo? The embryo is chosen according to a number of factors, which include the shape and number of cells and how quickly they are dividing. More than 50% of IVF embryos are chromosomally abnormal, so are weeded out.

What are the rules on the numbers of embryos transferred? In January 2004 the Human Fertilisation and Embryology Authority limited to two the number of embryos that could be transferred in women aged under 40. For older women the limit is three. The majority of women under 40 opt for a double embryo transfer. Sweden permits the transfer of two embryos only in exceptional circumstances, while Finland, Belgium and the Netherlands routinely transfer single embryos.

Is a change in the rules likely? Fertility experts believe Britain is moving towards single-embryo transfers. The HFEA set up a panel of clinicians, embryologists and patient representatives to look at the question. Single-embryo transfers could cover younger women and those susceptible to pre-eclampsia or diabetes, both complications of multiple births. But, with treatment costing £3,000-£6,000, patients tend to believe the more embryos the better. The authority is expected to report on its study next year.